This invention relates in general to electric motor systems and in particular to controllers for brushless motors.
Brushless dc motors are rising in popularity. Originally intended for servo applications and robotics, these motors are now replacing ac motors and conventional dc motors in high-speed applications, slow-speed direct-drive applications and any other applications that require speed control.
A brushless dc motor typically includes a permanent magnet rotor surrounded by stator windings. A controller commands an inverter to energize the stator windings in a sequence that creates a rotating stator magnetomotive (mmf) vector. The resulting torque reaction between the stator mmf vector and the rotor's flux vector causes the rotor to rotate. Motor torque is proportional to the amount of current flowing through the windings. The current is regulated by pulse-width modulating the solid-state switches of the inverter.
The inverter is typically operated in a current-controlled mode. In this mode, only two-phase windings are energized at any given time, except at the "commutation time" when current in one phase winding is being reduced to zero. Commutation information is derived from rotor position sensors or in some cases, the so-called back emf of the motor. This commutation information allows the controller to energize the windings in the correct sequence. The current-controlled motor of operation is preferred because the motor is simple to control.
Several disadvantages of operating the inverter in the current-controlled mode become apparent with increases in the rotational speed of the motor rotor. As the rotor rotates, its magnetic field interacts with the stator windings causing a back emf to be generated. As motor speed increases, a phase shift develops between the current in the phase windings and motor back emf. This phase shift reduces the effective power factor at which the motor operates. To correct for this reduction, the effective inductance (referred to as the commutation inductance) of the machine can be reduced; however, such a reduction increases peak-to-peak ripple in the stator windings requiring a higher KVA rating of the inverter. To reduce the ripple, the switching frequency can be increased. However, switching losses are proportional to switching frequency; therefore, efficiency for the drive would be reduced.